SIL — Safety Integrity Level

A Safety Integrity Level is a discrete grade — SIL 0 to SIL 4 — expressing how demanding the safety requirements on a function or system are, and therefore how rigorous its development and assessment must be. In the railway context the grades come from the CENELEC chain: EN 50126 defines the RAMS process in which safety requirements are allocated, EN 50129 applies them to electronic signalling systems, and EN 50716 prescribes, for each software SIL, which lifecycle activities, techniques and independence of roles are required. SIL 4 marks the most critical functions — interlocking logic, ETCS movement authority handling — where a dangerous failure could directly cause a catastrophic accident; SIL 0 covers functions with no safety impact beyond basic quality. A SIL attaches to a specific function against a quantified tolerable hazard rate, not to a product in general: 'a SIL 4 platform' is marketing shorthand for 'a platform assessed as capable of hosting SIL 4 functions'. Allocation happens during risk analysis (EN 50126-2, and SUBSET-091 for ETCS), and the resulting evidence is presented in the safety case that an independent assessor reviews.